The invention relates to the investigation of formations traversed by a borehole by means of a sonde equipped with measuring pads.
To carry out certain measurements in boreholes, and in particular for dip measurements, pads are applied against the borehole walls, these pads being equipped with electrodes or transducers carried by arms articulated on a sonde body member and distributed symmetrically around the latter. The sonde body member is connected to an upper part or a cartridge which is suspended from a cable allowing electrical transmission to surface equipment. Resilient means act on the arms so as to apply the pads against the walls with a suitable pressure, and a system controlled from the surface equipment makes it possible to retract the arms along the sonde body member against the action of the resilient means. The pads are placed in contact with the walls only during measurement movements.
U.S. Pat. No. 3,685,158 (J. Planche) describes a dipmeter apparatus with four arms associated in two independent pairs, each pair comprising two opposite arms whose movements are linked. The opposite pads are thus always spread symmetrically in relation to the sonde body member, but the pads of one pair can be spread differently in relation to the pads of the other pair. This arrangement allows the pads to remain applied against the walls in the case of an oval hole, the pads then forming (view in plan) the apexes of a diamond in the center of which is located the sonde body member.
Furthermore, in the above-mentioned apparatus, the pads are forced to remain coplanar so as to simplify the processing of measurement signals. In other words, the pads can move laterally in relation to the body member only in a plane perpendicular to the sonde axis. This is accomplished by mounting the pads on slides carried by the arms and connecting them to the sonde body member by linkage systems in the form of a Y.
In deviated boreholes, the weight of the apparatus has a radial component with acts on the lower pad(s) against the action of the resilient means so that the upper pad(s) symmetrical in relation to the lower pad(s) have a tendency to leave the wall, thereby deteriorating the corresponding measurement signals. To reduce the effect of borehole inclination, it is possible, as provided for in U.S. Pat. No. 3,423,671 (A. Vezin), to mount elastic centering devices at the center of gravity of the apparatus. However, the compensation obtained is only partial and these centering devices, constantly in contact with the walls, undergo rapid wear. They can, moreover, be used only in boreholes of sufficient diameter.
U.S. Pat. No. 3,474,541 (W. E. Cubberly, Jr.) describes a caliper logging apparatus adapted to deviated boreholes in which an articulated joint connects the upper part (connected to the cable) to the sonde. The sonde has four arc springs arranged in two opposite pairs, the ends of which are connected to the sonde while remaining mobile along the longitudinal direction of the sonde. The movements of the ends of each pair of springs are measured to determine the size of the hole along two perpendicular directions. This known apparatus does not comprise pads kept parallel to the axis of the sonde, and hence differs basically from the invention.